Origin of the Earth's crust

The Earth's earliest oceanic crust, which was probably worldwide in extent,appeared by ≈ 4.5 Ga and likely formed at ocean ridges. It may have been comprised chiefly of komatiite, with basalt becoming more important with time. The earliest continental crust appears to have formed by ≈ 4 Ga in response to wet partial melting of mafic sources in descending slabs. It was of tonalite-granodiorite composition and of local extent. Continental crust has grown by magma additions, thrust stacking, accretion of arcs, and welding of sedimentary prisms to continental margins. Periods of rapid continental growth are recorded at 2.7-3.0 Ga and 1.7-1.9 Ga, the former of which is most significant and may reflect catastrophic, eclogite-driven subduction of basalt. Well documented secular changes that reflect one or a combination of (1) cooling of the Earth, (2) rapid continental growth in the late Archean, or (3) reworking of the continents, include a decrease in komatiite or after 2.7 Ga, a long-term change in ⁸⁷Sr/⁸⁶Sr in marine carbonates, a decrease in Ni-Cr in pelites after 2.5 Ga, changes in the composition of greenstone volcanic rocks, and the increasing importance of alkaline igneous rocks and blueschists after 1 Ga. U-Pb zircon dates from igneous rocks do not support worldwide episodicity or synchronicity of orogeny. All of the terrestrial planets may have had magma oceans and komatiite-driven plate tectonics at ≈ 4.5 Ga. Differences in planetary crustal histories largely reflect different cooling rates of the planets with only Earth and possibly Venus cooling slowly enough to sustain plate tectonics.